A semiconductor laser includes a body of semiconductor material, generally composed of group III-V compounds, having a thin active layer between layers of opposite conductivity type, i.e., a layer of P-type conductivity on one side of the active layer and a layer of N-type conductivity on the other side of the active layer. Such a laser, however, typically emits light in more than one optical mode which limits its utility. Botez, in U.S. Pat. No. 4,215,319 issued July 29, 1980 and entitled SINGLE FILAMENT SEMICONDUCTOR LASER, has disclosed a laser having a stable, single mode, output light beam. The control over the output light beam from this laser arises from the tapering in thickness of the layers. This laser is prepared by deposition of the confinement and active layers onto a substrate having a pair of substantially parallel grooves therein. The tapering is caused by the difference in growth rate of the layers over a land between the grooves and over the grooves when the layers are prepared by liquid or vapor phase epitaxy techniques.
However, if the layers are deposited on an indium phosphide substrate having such a pair of parallel grooves, using either liquid phase or vapor phase epitaxy, flat, planar surfaces are observed with the grooves filling faster than the flat substrate portions until a continuous, smooth surface is obtained. This growth habit of InP limits the utilization of the structure disclosed by Botez for lasers composed of InP and related alloys. It would be desirable to have a laser composed of InP and related alloys which exhibits the tapered layer structure characteristic of the laser disclosed by Botez.